When a decal is applied over a hole or opening, like, say, the red decals applied over the Spitfire leading edge gun openings...
...some cracking or shattering might be locally experienced when trying to pierce-open the underlying hole (or any kind of underlying opening, like an underwing shell ejector chute).
What I do is dip a toothpick into decal softener...
...then carefully pierce through the opening...
The softener moistened tip will gently soften the decal locally, so it can be pierced without cracking...
